A membrane consisting of a macroporous support and a membrane layer is a composite membrane. The macroporous support contributes in here to the mechanical strength of the total membrane. Whenever the membrane layer with a high pore density is deposited relatively thin in comparison to the mean pore size on a relatively thick support, a membrane with a high flux is obtained. In case the inorganic membrane layer as well as the inorganic support is made of suitable ceramic materials then the membrane will be chemically highly inert and will operate well at elevated temperatures.
From EP-A-0-144 097 a composite ceramic membrane is known having the above-mentioned characteristics. This ceramic membrane has a membrane layer that is formed by coating a porous support with a thin film by immersion in a stable sol of colloidal metal oxide (g -AlOOH) particles and then drying and heating the support, thus forming a macroporous membrane layer of metal oxide particles.
Such known inorganic membranes may display mechanical faults, like little fractures and `pinholes` with typical size 10 .mu.m to 100 .mu.m, which are relatively large in comparison to the mean pore size, typically 5 nm-5 .mu.m, thus reducing the separating characteristics.
Another intrinsic disadvantage of these membranes is the relatively large thickness of the membrane layer in comparison to the mean pore size, resulting in reduced flux characteristics of the membrane. In said patent publication the thickness of the membrane layer ranges from 50 to 1000 times the mean pore size of the membrane layer.